Method and apparatus for performing sequential closed loop multiple input multiple output (MIMO)

ABSTRACT

In a communication system using closed loop MIMO, beam forming information may be fed back from a receiver to a transmitter sequentially over a number of frames. The beam forming matrices that are fed back may be quantized.

TECHNICAL FIELD

The invention relates generally to wireless communication and, moreparticularly, to channel training techniques and structures for use inwireless systems.

BACKGROUND OF THE INVENTION

Multiple input multiple output (MIMO) is a radio communication techniquein which both a transmitter and a receiver use multiple antennas towirelessly communicate with one another. By using multiple antennas atthe transmitter and receiver, the spatial dimension may be takenadvantage of in a manner that improves overall performance of thewireless link. MIMO may be performed as either an open loop or a closedloop technique. In open loop MIMO, a transmitter has no specificknowledge of the condition of the channel before signals are transmittedto a receiver. In closed loop MIMO, on the other hand, channel-relatedinformation is fed back from the receiver to the transmitter to allowthe transmitter to precondition transmit signals before they aretransmitted to better match the present channel state. The amount offeedback information that is delivered from a receiver to a transmitterin a system using closed loop MIMO can be very large. This may beparticularly true in closed loop MIMO systems that utilize singularvalue decomposition (SVD) techniques in the receiver. There is a generalneed for strategies to reduce the overall amount of feedback used in aclosed loop MIMO system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example wireless communicationlink in a MIMO-based wireless system in accordance with an embodiment ofthe present invention;

FIG. 2 is a block diagram illustrating an example transmitterarrangement that may be used in an SVD-MIMO based system in accordancewith an embodiment of the present invention;

FIG. 3 is a timing diagram illustrating an example wireless frameexchange sequence between an initiator and a responder in a wirelessnetwork in accordance with an embodiment of the present invention; and

FIG. 4 is a block diagram illustrating an example transmitterarrangement in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention. It is to be understood that the variousembodiments of the invention, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described herein in connection with one embodiment may beimplemented within other embodiments without departing from the spiritand scope of the invention. In addition, it is to be understood that thelocation or arrangement of individual elements within each disclosedembodiment may be modified without departing from the spirit and scopeof the invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined only by the appended claims, appropriately interpreted, alongwith the full range of equivalents to which the claims are entitled. Inthe drawings, like numerals refer to the same or similar functionalitythroughout the several views.

FIG. 1 is a block diagram illustrating an example wireless communicationlink 10 in a MIMO-based wireless system in accordance with an embodimentof the present invention. As illustrated, a wireless initiator device 12is communicating with a wireless responder device 14 via a wirelesschannel. The initiator device 12 has three transmit antennas 16, 18, 20and the responder device 14 has two receive antennas 22, 24. Thewireless channel is a multiple input, multiple output (MIMO) channel.Although illustrated with three transmit antennas 16, 18, 20 and tworeceive antennas 22, 24 in FIG. 1, it should be appreciated that anynumber (i.e., greater than 1) of transmit antennas and receive antennasmay be used to form a MIMO channel. During a wireless frame exchangesequence, the wireless initiator device 12 may transmit user data to theresponder device 14. After receiving a data frame from the initiator 12,the responder 14 may transmit an acknowledgement (ACK) frame (and/orother information) back to the initiator 12 to acknowledge that the dataframe was successfully received. The same antennas may or may not beused for the reverse direction link that were used for the forwarddirection link.

The wireless link 10 of FIG. 1 may utilize closed loop MIMO techniques.That is, the responder 14 may transmit channel-related feedbackinformation to the initiator 12 for use by the initiator 12 indeveloping subsequent transmit signals. By utilizing knowledge of thechannel, the initiator 12 can tailor the transmit signal to the channelin a manner that simplifies receiver processing and/or improves receiverperformance in the responder 14. The responder 14 can generatechannel-related feedback information by appropriately processingtraining signals received from the initiator 12. Various methods ofdeveloping such channel-related information are known in the art. Onemethod of developing channel-related feedback information makes use of amathematical technique known as singular value decomposition (SVD). WhenSVD is utilized in a MIMO-based system, the overall technique may bereferred to as SVD-MIMO. To facilitate understanding and simplifynotation, the discussion that follows will be with respect to a singlesubcarrier in a multi-carrier system (e.g., an OFDM system). It shouldbe appreciated, however, that the below described functions willtypically need to be performed for each of the subcarriers within amulti-carrier system.

In a MIMO-based system, a wireless channel may be characterized using ann_(RX)×n_(TX) channel matrix H, where n_(RX) is the number of receiveantennas and n_(TX) is the number of transmit antennas. Using SVD, thechannel matrix H may be assumed to be in the form:

H=UΣV^(H)

where U and V are unitary matrices (i.e., matrices with orthonormalcolumns and unit amplitude), Σ is a diagonal matrix, and V^(H) is theHermitian of matrix V. A unitary matrix U has the following property:

U^(H)U=I

where I is the identity matrix. If the channel matrix H is in the aboveform, and if the matrix V can be determined, then the vector X ofcomplex symbols to be transmitted by the transmitter into the MIMOchannel may be multiplied by V before transmission. The transmitter willthus transmit symbols Z=VX, where V may be referred to as the beamforming matrix. The transmitted symbols Z are then acted upon by thechannel H and are also subject to noise in the channel. Thus, the signalY received by the receiver (at the other side of the MIMO channel) maybe represented as:

Y=HVX+N

where N is the additive noise. From the channel expression given above,it is found that:

HV=UΣV^(H)V=UΣI=UΣ

Therefore, Y may be expressed as:

Y=UΣX+N

In the receiver, the received signal Y can simply be matrix multipliedby U^(H) and the following result may be achieved:

YU ^(H) =UU ^(H) ΣX+U ^(H) N=IΣX+U ^(H) N=ΣX+U ^(H) N

Thus, if the diagonal matrix Σ is known, the symbols X may be recovered.The above-described technique essentially diagonalizes the channel andallows the originally transmitted symbols to be recovered in thereceiver. The elements of the diagonal matrix Σ are known as thesingular values (or eigenvalues) of the channel matrix H and they may bedetermined using well known SVD techniques.

The receiver associated with a MIMO channel will typically measure the Hmatrix using known training signals received from the transmitter. AnSVD may then be performed to determine the V matrix. In a closed loopsystem, the V matrix may then be transmitted back to the transmitter.The amount of V matrix data will often be quite large. For example, in asystem using orthogonal frequency division multiplexing (OFDM), the Vmatrix may include a 4×4 matrix for each of the subcarriers of an OFDMsymbol. As will be appreciated, this is a large amount of data to betransmitted back to the transmitter and may have a significant impact onoverall throughput within the system. In accordance with at least oneembodiment of the present invention, a sequential method of closed loopMIMO is provided that is capable of reducing the overall amount offeedback data that is transmitted back to the transmitter whenperforming SVD-MIMO. Instead of transmitting the entire V matrix back tothe transmitter for each frame received therefrom, the V matrixinformation may be transmitted back sequentially over a number of framesso that the average amount of feedback is significantly less.

FIG. 2 is a block diagram illustrating an example transmitterarrangement 30 that may be used in an SVD-MIMO based system inaccordance with an embodiment of the present invention. The transmitterarrangement 30 may be located within, for example, a wireless devicethat is configured to act as an initiator device within a highthroughput wireless network. Other applications also exist. As shown,the transmitter arrangement 30 may include one or more of: a spatialstream interleaver 32, a beamformer 34, a number of inverse fast Fouriertransform (IFFT) devices 36, 38, 40, and a number of antennas 42, 44,46. The spatial stream interleaver 32 receives data symbols at an inputthereof and separates these data symbols into a plurality of spatialstreams 48. The data symbols may be received by the spatial streaminterleaver 32 from, for example, a mapper unit (not shown) that mapsinput data into corresponding modulation symbols based on apredetermined modulation scheme (e.g., binary phase shift keying (BPSK),quadrature phase shift keying (QPSK), quadrature amplitude modulation(QAM), etc.). The beam former 34 receives the spatial streams from thespatial stream interleaver 32 and matrix multiplies a present vector ofsymbols by the beam forming matrix V to generate signals for delivery tothe multiple transmit antennas 42, 44, 46. The number of transmitantennas may or may not be equal to the number of spatial streams inputto the beam former 34.

As OFDM is being used in the illustrated embodiment, the output signalsof the beamformer 34 may each be processed by an IFFT 36, 38, 40 beforebeing transmitted by a corresponding transmit antenna 42, 44, 46. Thebeam forming matrix V used by the beamformer 34 is derived from feedbackinformation received from a device on the other side of the MIMO channel(e.g., a responder device, etc.). As will be appreciated, thearchitecture of the transmitting arrangement 30 of FIG. 2 represents onepossible transmitter architecture that may be used in accordance withthe present invention. Other architectures may alternatively be used.

FIG. 3 is a timing diagram illustrating an example wireless frameexchange 50 that may occur in a wireless network using MIMO inaccordance with an embodiment of the present invention. The upperportion 52 of the diagram represents the transmit activity of aninitiator device during the wireless frame exchange 50 and the lowerportion 54 represents the transmit activity of a responder device. Asshown, the initiator initially transmits a request to send (RTS) frame56 to the responder. The RTS frame 56 may include information such as,for example, the address of the initiator, the address of the subjectresponder, and the duration of the frame exchange to follow. The RTSframe 56 may also include training signals for use in performing channeltraining in the responder. When the responder receives the RTS frame 56,it processes the received training signals to develop channelinformation that characterizes the MIMO channel. After a short period(e.g., a short inter frame space (SIFS)), the responder may transmit aclear to send (CTS) frame 58 back to the initiator indicating that it isclear to start transmitting data. The CTS frame 58 may include channelrelated feedback information (e.g., a beam forming matrix) for use bythe initiator in transmitting data.

The CTS frame 58 may also include the same duration information that theRTS frame 56 included (or a slightly modified version). Any otherdevices receiving either the RTS frame 56 or the CTS frame 58 may readthe duration information and set a network allocation vector (NAV) basedthereon. These other devices will thereafter treat the wireless mediumas reserved until the end of the identified duration and refrain fromtransmitting. In this manner, collisions may be avoided.

The initiator receives the CTS frame 58 and determines that it may nowstart to transmit data. The initiator reads the feedback informationwithin the CTS frame 58 and uses the information to generate (after aSIFS) a data frame 60 for transmission to the responder. In addition todata, the data frame 60 may also include channel training signals. Theresponder may receive the data frame 60, read and record the user datatherein, and use the training signals to again generate channel relatedinformation. The responder may then transmit a response frame 62 back tothe initiator that includes an acknowledgement packet acknowledging thereceipt of the data frame 60 and also new channel related feedbackinformation. This process may be repeated with additional data frames(e.g., frames 64, 68, etc.) and additional response frames (e.g., frames66, 70, etc.) until all of the relevant data has been successfullytransferred to the responder. The final response frame 70 may notinclude feedback information.

In conceiving the present invention, it was determined that successiveapproximations of an optimal SVD beam forming matrix can converge tonear optimal SVD-MIMO performance, while significantly reducing thefeedback required for convergence and subsequent tracking of a dynamicchannel. The coherence time of a channel in a wireless network is oftenlong. For example, the coherence time in an IEEE 802.11 based networkmay be in the hundreds of milliseconds. A frame in such a network (e.g.,a physical layer protocol data unit (PPDU)), on the other hand, may beon the order of I millisecond. The channel coherence time, therefore,may be at least several frame exchange sequences in length. Based on theabove, it was determined that it was possible to utilize quantizationmethods with respect to the beamforming V matrices with little impact onlink performance. By quantizing the V matrix information that is to befed back to the initiator, the overall amount of feedback informationmay be reduced considerably. The feedback information may be transmittedback to the initiator over several frames, rather than all at once. Aswill be described in greater detail, the feedback matrix that isdelivered to the initiator from the responder in response to eachreceived data frame may be a correction matrix to the previously used Vmatrix, rather than the entire V matrix, in a differential encodingstyle approach.

With reference to FIG. 3, in at least one embodiment of the presentinvention, the initiator may use a predetermined matrix (e.g., anidentity matrix, I) as the beam forming matrix V₀ during transmission ofthe RTS frame 56. When the responder subsequently receives the RTS frame56, it may calculate the channel matrix H₀ of the MIMO channel. Theresponder may then perform an SVD operation on the channel matrix H₀ todetermine a corresponding beam forming matrix {tilde over (V)}₀ to befed back to the initiator. In effect, the matrix {tilde over (V)}₀ is acorrection to the beam forming matrix V₀ that was used by the initiatorto transmit the RTS frame 56 to the responder (which, as discussedabove, may be the identity matrix). In at least one embodiment, asdescribed above, quantization is used to describe the beamformingmatrices in the network. Any quantization technique may be usedincluding, for example, a coarse element-by-element type quantization, avector type quantization (e.g., Grassmanian beam forming, etc.), and/orothers.

The initiator receives the beam forming matrix {tilde over (V)}₀ anduses it to update the beam forming matrix V₀ used to transmit the RTSframe 56 for use with the subsequent data frame 60. This update may be asimple matrix multiplication (e.g., a right multiplication). Theinitiator may then use the new beam forming matrix V₁ to transmit dataframe 60. The responder receives data frame 60 and determines thechannel matrix of the channel. However, the channel matrix determined bythe responder will be for the combined channel, including both the beamforming matrix V₁ and the actual channel H₁ (i.e., {tilde over(H)}₁=V₁H₁). The responder then performs an SVD operation to determine abeam forming matrix {tilde over (V)}₁ to be fed back to the initiator.The beam forming matrix {tilde over (V)}₁ is what the responder wouldwant the initiator to precondition the channel with, assuming thecombined channel is the actual channel. Quantization techniques areagain used. If the beam forming matrix V₁ used to transmit data frame 60had been optimal, then the SVD operation would result in a diagonalmatrix and there would be no feedback data to be transmitted. However,because quantization is being used, and because of the effects ofchannel fading, an ideal beam forming matrix may rarely be achieved.

As before, the initiator receives the beam forming matrix {tilde over(V)}₁ and uses it to update the beam forming matrix V₁ used to transmitdata frame 60 (e.g., V₂=V₁{tilde over (V)}₁=V₀{tilde over (V)}₀{tildeover (V)}₁). The initiator then uses the new beam forming matrix V₂ totransmit data frame 64, and so on. In general, the beam forming matrixfor the kth data frame may be expressed as:

$V_{k} = {{V_{k - 1}{\overset{\sim}{V}}_{k - 1}} = {\prod\limits_{\kappa = 0}^{k}{\overset{\sim}{V}}_{\kappa}}}$

where the identity matrix was used as the initial beam forming matrixV₀.

FIG. 4 is a block diagram illustrating an example transmitterarrangement 80 in accordance with an embodiment of the presentinvention. As illustrated, the transmitter arrangement 80 includes: abeam former 82, a plurality of transmit antennas 84, 86, 88, first andsecond beam forming matrix storage areas 90, 92, and a combiner 94. Thebeam former 82 receives data symbols at inputs thereof, via multiplespatial streams, and matrix multiplies vectors of input symbols by thebeam forming matrix V_(i). The outputs of the beam former 82 then feedthe multiple transmit antennas 84, 86, 88. Although not shown, otherfunctionality may be between the beam former 82 and each individualtransmit antenna 84, 86, 88 (e.g., an IFFT, a power amplifier, etc.).The transmit antennas 84, 86, 88 may include any type of antenna elementincluding, for example, dipoles, patches, helical antennas, and/orothers. Any number of transmit antennas may be used (n_(TX)>1).

The first beam forming matrix storage area 90 is operative for storingthe beam forming matrix that was used to transmit the last data frametransmitted by the transmitter arrangement 80 (i.e., V_(i−1)). Thesecond beam forming matrix storage area 92 is operative for storing thebeam forming correction matrix most recently received from the responder(i.e., {tilde over (V)}_(i−1)). The combiner 94 is operative forcombining the stored matrices to generate an updated beam forming matrixV_(i) for use by the beam former 82. In at least one embodiment, thecombiner 94 is a matrix multiplication unit. The first and second beamforming matrix storage areas 90, 92 may be associated with any type ofdevice that is capable of storing digital data. After the updated beamforming matrix V_(i) has been generated and delivered to the beam former82, it may then be stored within the first beam forming matrix storagearea 90 for use with a subsequent data frame. In at least oneembodiment, an initialization unit may be provided to initialize thebeam forming matrix that is used by the beam former 82 at the beginningof a frame exchange sequence (e.g., to the identity matrix, I). Asdescribed previously, quantization may be used for the beam formingmatrices. The transmitter arrangement 80 of FIG. 4 may be used, forexample, during a frame exchange sequence, such as the one illustratedin FIG. 3. Other architectures may alternatively be used.

V-matrix quantization can be achieved in a number of ways. A directmethod is to simply quantize element-by-element. A more efficientapproach is to apply vector-quantization techniques to the entirematrix. These methods may achieve quantization efficiency by exploitingproperties of unitary matrices in general, or SVD properties morespecifically. In particular, the U and V matrices are not unique. IfH=UΣV^(H) and D is a diagonal unitary matrix (that is; a diagonal matrixwith diagonal elements that are unit magnitude complex numbers), then,since diagonal matrices commute, it follows that(UD)Σ(VD)^(H)=UDΣD^(H)V^(H)=UDD^(H)ΣV^(H)=UΣV^(H)=H. Thus, the pair (UD,VD) provides another SVD decomposition. This invariance with respect todiagonal unitary matrices provides degrees of freedom that can beexploited in vector quantization. In addition to unitary matrixproperties, one can exploit the typically strong correlation betweenadjacent subcarriers in an OFDM system. One quantized V-matrix may beapplied to groups of adjacent OFDM subcarriers. Other quantizationtechniques may alternatively be used.

In at least one aspect, the present invention is based on convergencetowards SVD-MIMO over several packet exchanges. The quantization mayvary from packet to packet in order to facilitate rapid convergence(i.e., adaptive quantization). The first packet may utilize a fairlycoarse quantization followed by finer quantization on later packets.Thus, principles of differential-encoding (quantization) can be appliedas well.

In the description above, various features of the invention aredescribed using terminology (e.g., RTS, CTS, etc.) that is associatedwith the IEEE 802.11 wireless networking standard. It should beappreciated, however, that the invention is not limited to use withinsystems following the IEEE 802.11 standard and its progeny. Also, itshould be understood that the frame exchange sequence 50 of FIG. 3 is anexample of one possible application of sequential closed loop MIMO inaccordance with an embodiment of the invention. Many other applicationsalso exist. For example, the feedback delivered to a transmitter unitdoes not have to be made part of an acknowledgement frame. Any type offeedback path may be used. Similarly, RTS and CTS frames 56, 58 are notrequired. The inventive techniques and structures may be used inwireless networks and in other forms of wireless communication systems.

It should be appreciated that the individual blocks illustrated in theblock diagrams herein may be functional in nature and do not necessarilycorrespond to discrete hardware elements. For example, in at least oneembodiment, two or more of the blocks in a block diagram (e.g., beamformer 82 and combiner 94 in FIG. 4, etc.) may be implemented insoftware within a single (or multiple) digital processing device(s). Thedigital processing device(s) may include, for example, a general purposemicroprocessor, a digital signal processor (DSP), a reduced instructionset computer (RISC), a complex instruction set computer (CISC), a fieldprogrammable gate array (FPGA), an application specific integratedcircuit (ASIC), and/or others, including combinations of the above.Hardware, software, firmware, and hybrid implementations may be used.

In the foregoing detailed description, various features of the inventionare grouped together in one or more individual embodiments for thepurpose of streamlining the disclosure. This method of disclosure is notto be interpreted as reflecting an intention that the claimed inventionrequires more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive aspects may lie in less thanall features of each disclosed embodiment.

Although the present invention has been described in conjunction withcertain embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art readily understand.Such modifications and variations are considered to be within thepurview and scope of the invention and the appended claims.

1. A method comprising: receiving a signal Y_(i) from a MIMO channel,said signal Y_(i) including data symbols X_(i) that were matrixmultiplied by a beam forming matrix V_(i) within a remote transmitterbefore being transmitted into said MIMO channel, said MIMO channelhaving a channel matrix H_(i); using said signal Y_(i) to determine acombined channel {tilde over (H)}_(i) that includes effects of both thebeam forming matrix V_(i) and the channel matrix H_(i); performing asingular value decomposition (SVD) of the combined channel {tilde over(H)}_(i) to determine a beam forming matrix {tilde over (V)}_(i)representing a correction that is needed for the beam forming matrixV_(i); and transmitting said beam forming matrix {tilde over (V)}_(i) tosaid remote transmitter to be combined with said beam forming matrixV_(i) to generate a new beam forming matrix V_(i+1) for use in asubsequent data transmission from said remote transmitter.